The practice one takes up when skullfucking becomes tedious. Skullfucking is the easiest of the two acts, as you only have to remove your partners eye to get somewhere to stick your thingy. A proper squicking requires you to trephine your partner (make a hole in your partners skull) and pork its brain this way. Aiming for the gap between the two hemispheres is said to provide you with firestorming orgasms.

To skull fuck someone, not limited to the eye socket but any part of cranuim.

Squicking his bosses daughter right there in front of him was his revenge.
The bad man chisled a piece of her forehead away with a rusty screwdriver so he could squick her.(Warning: Squicking someone can kill them)